1. Field of the Invention
The present invention relates to an image processing apparatus and control method thereof and, more particularly, to an image processing apparatus for suppressing color crosstalk in an image sensed by an image sensor, and a control method thereof.
The present invention also relates to an image-capturing apparatus using the image processing apparatus according to the present invention.
2. Description of the Related Art
An image sensor mounted in an image-capturing apparatus such as a digital camera has an arrangement in which pixels each having a photoelectric conversion element are arrayed in a matrix. The image sensor converts an object image formed via an optical system into an electrical signal for each pixel. A problem in the image sensor having this arrangement is “color crosstalk” generated when obliquely incident light enters not only a predetermined pixel but also an adjacent pixel.
The degree of color crosstalk changes depending on the pixel color (color of a color filter arranged in a pixel). Color crosstalk becomes large in a pixel of another color adjacent to a color filter of a high-transmittance color (generally green). For example, in an image sensor having color filters of a general Bayer array, color crosstalk caused by light which obliquely enters a green pixel out of red, green, and blue pixels, passes through the green color filter, and then enters an adjacent blue or red pixel becomes relatively large. The color crosstalk relatively increases the sensitivity of a blue or red pixel adjacent to a green pixel with respect to the green wavelength, and relatively decreases that of a green pixel.
As a technique for canceling color crosstalk, Japanese Patent Laid-Open No. 10-271519 discloses a technique of storing a correction coefficient determined based on the color crosstalk component, and correcting, using the correction coefficient, a color crosstalk component contained in an image signal output from an image sensor.
Japanese Patent Laid-Open No. 2009-188461 discloses a technique of changing the color crosstalk correction amount based on one of the aperture value, zoom position, and image capturing sensitivity setting in image capturing.
Conventionally, techniques related to preparing in advance a correction coefficient for correcting generated color crosstalk and correcting an image signal have been used as disclosed in Japanese Patent Laid-Open Nos. 10-271519 and 2009-188461. These methods correct an image signal by applying, to each color pixel, a gain for correcting a relative sensitivity difference caused by color crosstalk. When color crosstalk is large (sensitivity difference is large), a large gain must be applied. This may decrease the S/N ratio or degrade the image quality due to overcorrection and undercorrection. When different gains are applied for respective, red, green, and blue pixel colors, the luminance changes upon a change of a luminance composition ratio given by red:green:blue=3:6:1. Color crosstalk correction changes the exposure of an image to be captured.
Further, when the correction gain is applied to all the pixels of an image signal, the time taken for correction processing becomes longer as the number of pixels of the image sensor becomes larger.